Diamond does not actually tell us why the major five were able to spread and the minor nine were not. It is not particularly relevant to his thesis. More relevant to his thesis is the fact that 13 of the "Ancient Fourteen" were found in Eurasia.

So, to answer your question, we need to speculate. The most likely answer is that the minor nine were in some way less suited to life in a broad range of areas. We can see that this is certainly true with the camel species, which live in deserts and the reindeer, which is an arctic animal. The water buffalo , of course, needs a wet environment.

The most likely explanation, then, is that the major five were more adaptable to a wider variety of environments and were therefore able to spread across the world.